The present invention relates to powder materials for use in powder plasma build-up welding for the purpose of imparting corrosion resistance and abrasion resistance to furnace pipes, furnace wall panels, or the like, in industrial recovery boilers in paper mills or the like. The present invention also relates to metals formed by powder plasma build-up welding.
In industrial recovery boilers, a material for build-up welding containing 18% Cr has been used on the surface of members being subjected to build-up welding in a furnace pipe panel, which is a combination of a plurality of carbon steel pipes, in view of corrosion resistance and abrasion resistance under the corrosive environment of smelt at a high temperature. As the build-up material, covered electrodes for shielded arc welding containing 18% Cr and wires for gas-shielded arc welding (hereinafter referred to as xe2x80x9cMAG welding wiresxe2x80x9d) containing 18% Cr have been conventionally used.
FIG. 1 shows basic constitution of a conventionally known device for powder plasma build-up welding. A pilot arc power source 10 is connected between a tungsten electrode 1 and a nozzle 2, and a direct current power source 12 is connected between the tungsten electrode 1 and a base metal 11. An intermediate nozzle 3 is disposed outside the nozzle 2, and an outer nozzle 4 is disposed outside the intermediate nozzle 3. Under the conditions in which a plasma gas (for example, Ar gas) is supplied between the tungsten electrode 1 and the nozzle 2, a powder material for build-up welding together with a feed gas (for example, Ar gas) is supplied between the nozzle 2 and the intermediate nozzle 3, and a shielding gas (for example, Ar gas) is supplied between the intermediate nozzle 3 and the outer nozzle 4, a plasma arc is generated between the nozzle and the base metal so that the powder material supplied can be fused, and deposited as a welded metal 13 on the base metal 11.
FIG. 2 shows a situation in which powder plasma welding is being performed on a carbon steel pipe. That is, a plasma arc is generated between the carbon steel pipe 5 (for example, JIS STB410) and a plasma welding torch 6 which faces the carbon steel pipe 5, and a powder material supplied is fused in sequence to form a molten pool 7. By gradually moving the plasma welding torch 6, the molten pool 7 is solidified to form a welded metal.
A characteristic of the powder plasma build-up welding is that this method can achieve a maximum amount of deposition of 4 to 6 kg/1 hr in contrast to the shielded arc welding or MAG welding method, which can achieve a maximum amount of deposition of only 3 kg/1 hr. Thus, use of the powder plasma build-up method can realize highly efficient welding.
In addition, since the ratio of dilution with the base metal in the powder plasma build-up welding method is lower than that in present MAG welding methods, the powder plasma build-up method can limit decrease in the amount of Cr in the metal formed by build-up welding. Moreover, the powder plasma build-up method can provide a welded metal of high quality, which does not have welding defects such as blow holes and incomplete fusion.
In recent years, the corrosive environment under which recovery boilers operate has become more severe, as the concentration and the temperature of the materials dealt with therein have become higher. Since severe loss of built-up metal from a furnace pipe occurs in parts of conventionally welded portions having built-up metal of 18% Cr due to corrosion and abrasion by smelt, development of a material for build-up welding which exhibits superior corrosion resistance has been demanded.
It is anticipated that use of powder welding material containing 17 to 30% of Cr for plasma build-up welding will result in a lower rate of loss of built-up metal from furnace pipes due to smelt and a longer life for the furnace pipes.
When welding is performed for a panel which is a combination of a plurality of carbon steel pipes, build-up welding is performed only on the inner side of the furnace wall. Therefore, distortion occurs due to heat input. Accordingly, a stress relieving treatment is carried out on the panel after welding. However, in view of bending stresses due to conveyance and partial processing in the plant, development of welding material has been necessary which has good bending properties when it is untreated after welding in addition to good bending properties after the stress relieving treatment.
Accordingly, the present invention provides a powder material for powder plasma build-up welding which can exhibit excellent corrosion resistance and bending properties when it is subjected to a powder plasma build-up welding method, which is a highly efficient welding method, in build-up welding for imparting corrosion resistance and abrasion resistance to furnace pipes and furnace wall panels for recovery boilers.
As a result of diligent researches, the present inventors found that a welded metal which is obtained from a powder material for powder plasma build-up welding comprising C in an amount of 0.06 to 0.15%, Si in an amount of 0.2 to 1.0%, Mn in an amount of 0.2 to 1.0%, Cr in an amount of 17 to 30%, Nb in an amount of 0.6 to 1.5%, Ni in an amount of not more than 0.5%, and the balance consisting of Fe and unavoidable impurities has excellent corrosion resistance and bending properties.
According to the present invention, the Cr content is basically 17 to 30%. However, since the amount of corrosive loss of the built-up metal in a recovery boiler varies depending on heat load, conditions of operation, and location within the boiler, a material having a high Cr content (hereinafter occasionally referred to as xe2x80x9c25Cr materialxe2x80x9d) can be used where corrosive loss of the built-up metal is severe in consideration of corrosion resistance, while a material having a low Cr content (hereinafter occasionally referred to as xe2x80x9c18Cr materialxe2x80x9d) can be used where corrosive loss of the built-up metal is moderate, for the purpose of reducing cost.
That is, 25Cr material is characterized by comprising C in an amount of 0.06 to 0.15%, Si in an amount of 0.2 to 1.0%, Mn in an amount of 0.2 to 1.0%, Cr in an amount of 23 to 30%, Nb in an amount of 0.6 to 1.5%, Ni in an amount of not more than 0.5%, and the balance consisting of Fe and unavoidable impurities, and 18Cr material is characterized by comprising C in an amount of 0.06 to 0.15%, Si in an amount of 0.2 to 1.0%, Mn in an amount of 0.2 to 1.0%, Cr in an amount of 17 to 23%, Nb in an amount of 0.6 to 1.5%, Ni in an amount of not more than 0.5%, and the balance consisting of Fe and unavoidable impurities.
The powder material according to the present invention is not limited to one consisting exclusively of powder of an alloy which has the above composition, and is not particularly limited as long as the powder material is a combination of metal powder and alloy powder which finally provides the above composition, such as a mixture of metal powders of each constituent element and a mixture of alloy powders each of which consists of a plurality of elements. The powder material of the present invention may normally be prepared with a grain size of approximately 63 to 500 xcexcm for powder plasma build-up welding, however, this grain size is not a limitation of the present invention.